A vehicular lamp fitting that includes: a light guiding plate having a front surface and a back surface on the opposite side of the front surface; and a light source which emits light guided by the light guiding plate, is known (e.g. Japanese Laid-open Patent Publication No. 2018-006226).
In the vehicular lamp fitting according to Japanese Laid-open Patent Publication No. 2018-006226, a part of the light from the light source, that is guided inside the light guiding plate, is internally reflected by V grooves (extending in the horizontal direction), which are disposed on the back surface of the light guiding plate, and is exits through the front surface.